American presidents are not elected by a nationwide popular vote, though hypothetical nationwide matchups in voter surveys are the customary journalistic device for tracking the White House race. Presidents are chosen by a winner-take-all system in 51 separate contests - in the 50 states and the District of Columbia. The winner must capture 270 of the 538 electoral votes, apportioned to states according to their shares of House and Senate seats.

In 1980 the nationwide surveys were showing a close popular race between Carter and Reagan, but at the same time the electoral vote assessments showed Reagan way ahead - and the electoral picture proved the truer.

For 1984, both the national surveys and electoral approaches are showing a tight race. The ABC News-Washington Post poll has Reagan ahead of Mondale by eight points (51-43), Hart by four (49-45). The electoral vote count by party professionals shows a near-draw - 189 electoral votes leaning to the Democrats (Mondale or Hart), 187 to the GOP, with 162 votes a tossup. When it is observed that Reagan tends to run up a surplus of popular votes in the states where he is the strongest, in the West, his national survey lead can be discounted a couple of points.

Most striking about the electoral-vote picture is that the South and border-state region, the largest parcel of electoral votes with 174, is up for grabs. The race in seven states is rated dead even. Reagan is a little ahead in Oklahoma, and well ahead in Virginia. But he trails in five states and the District of Columbia. Some 99 of the South's electoral votes are a tossup, another 38 close to even; a Democratic ticket can count on some 25 votes, a GOP ticket 12.

Now that's close! And it should help explain much of Washington's current politics and the two parties' 1984 campaign approaches. As a broad configuration of issues, Mr. Reagan is emphasizing military strength, traditional and fundamentalist religious themes, Central American aid - all appealing to the white, male, conservative, nationalistic voters who constitute his base in the South. The Democrats are staying only a tad distant from Reagan on the same issues and are avoiding topics like Hispanic immigration for basically the same reasons - to keep competitive in the South.

The second major point about the electoral-vote race is that more of Reagan's electoral votes, some 128 of his 187, are secure. These are mostly in the mountain states, the northern Great Plains, and northern New England.

By contrast, most of a Democratic ticket's votes, 147 of the 189, are weak. Only 42 votes in places like Hawaii, Massachusetts, West Virginia, and Minnesota seem securely in the Democratic camp. This makes the Democrats' prospects considerably more risky than Reagan's, despite the closeness of the overall numbers.

Where else, outside the South, is the race close? Oregon, Iowa, Illinois, New Jersey, and Connecticut could go either way. So could Missouri, Michigan, Wisconsin, where Reagan now trails narrowly, or Ohio where he may have a slight edge. In the East, Reagan is somewhat behind in Pennsylvania, New York, and Delaware; continuing economic recovery could help him here, just as it has in some of the Midwest industrial states.

What the electoral vote map says is that both parties' resources will be heavily focused on the South this summer and fall. How happily Jesse Jackson's followers exit the Democratic convention in July will be crucial, given the black vote's significance in the South and in other close states, but it cannot come at the risk of alienating white votes by any sellout impression.

If things go right for Reagan, his more secure electoral vote position could magnify a narrow lead into a more decisive outcome. For the Democrats the imponderables are greater. But for either side, it is a victory that is still far from won.